Because of the increase in data intensive applications, the demand for bandwidth in communications has been growing tremendously. In response, the installed capacity of telecommunication systems has been increasing by an order of magnitude every three to four years since the mid 1970s. Much of this capacity increase has been supplied by optical fibers that provide a four-order-of-magnitude bandwidth enhancement over twisted-pair copper wires.
To exploit the bandwidth of optical fibers, two key technologies have been developed and used in the telecommunication industry: optical amplifiers and wavelength-division multiplexing (WDM). Optical amplifiers boost the signal strength and compensate for inherent fiber loss and other splitting and insertion losses. WDM enables different wavelengths of light to carry different signals parallel over the same optical fiber. Although WDM is critical in that it allows utilization of a major fraction of the fiber bandwidth, it would not be cost-effective without optical amplifiers. In particular, a broadband optical amplifier that permits simultaneous amplification of many WDM channels is a key enabler for utilizing the full fiber bandwidth.
Silica-based optical fiber has its lowest loss window around 1550 nm with approximately 25 THz of bandwidth between 1430 and 1620 nm. For example, FIG. 1 illustrates the loss profile of a 50 km optical fiber. In this wavelength region, erbium-doped fiber amplifiers (EDFAs) are widely used. However, as indicated in FIG. 2, the absorption band of a EDFA nearly overlaps its the emission band. For wavelengths shorter than about 1525 nm, erbium-atoms in typical glasses will absorb more than amplify. To broaden the gain spectra of EDFAs, various dopings have been added. For example, as shown in FIG. 3a, codoping of the silica core with aluminum or phosphorus broadens the emission spectrum considerably. Nevertheless, as depicted in FIG. 3b, the absorption peak for the various glasses is still around 1530 nm.
Hence, broadening the bandwidth of EDFAs to accommodate a larger number of WDM channels has become a subject of intense research. As an example of the state-of-the-art, Y. Sun et al. demonstrated in Electronics Letters, Vol. 33, No. 23, pp. 1965-67 (1997), a two-band architecture for an ultra-wideband EDFA with a record optical bandwidth of 80 nm. To obtain a low noise figure and high output power, the two bands share a common first gain section and have distinct second gain sections. The 80 nm bandwidth comes from one amplifier (so-called conventional band or C-band) from 1525.6 to 1562.5 nm and another amplifier (so-called long band or L-band) from 1569.4 to 1612.8 nm. As another example, M. Yamada et al. reported in Electronics Letters, Vol. 33, No. 8, pp. 710-711 (1997), a 54 nm gain bandwidth achieved with two EDFAs in a parallel configuration, i.e., one optimized for 1530-1560 nm and the other optimized for 1576-1600 nm. As yet another example, H. Masuda et al. reported in Electronics Letters, Vol. 33, No. 12, pp. 1070-72 (1997), a 52 nm EDFA that used two-stage EDFAs with an intermediate equalizer.
These recent developments illustrate several points in the search for broader bandwidth amplifiers for the low-loss window in optical fibers. First, bandwidth in excess of 40-50 nm require the use of parallel combination of amplifiers even with EDFAs. Second, the 80 nm bandwidth achieved by Y. Sun et al., may be very close to the theoretical maximum. The short wavelength side at about 1525 nm is limited by the inherent absorption in erbium, and long wavelength side is limited by bend-induced losses in standard fibers at above 1620 nm. Therefore, even with these recent advances, half of the bandwidth of the low-loss window, i.e., 1430-1530 nm, remains without an optical amplifier.